Revealing the Hidden Complexities of Fasting Blood Sugar

Apr, 2024

Everyone knows that fasting blood sugar, or blood glucose measured after an overnight fast, is a key indicator of diabetes risk. But new research is uncovering that this simple test may not tell the whole story. A large study harnessing continuous glucose monitoring has revealed significant day-to-day fluctuations in fasting blood sugar that could impact how diabetes is diagnosed. The findings challenge assumptions about what is considered “normal” and highlight the need to re-evaluate current diagnostic guidelines.

For decades, fasting blood sugar has served as a cornerstone test for screening and diagnosing diabetes. A single reading above 125 mg/dL places someone in the prediabetes range, while over 126 mg/dL meets the threshold for a diabetes diagnosis. But these cutoff points were established based on isolated snapshots of blood sugar, not its natural ebbs and flows. Continuous glucose monitoring may provide a more comprehensive view.

Scientists at the Weizmann Institute of Science in Israel analyzed data from over 8,000 non-diabetic adults who wore glucose sensors for two weeks as part of the large-scale 10K study. Their sensors automatically measured interstitial glucose levels every 15 minutes, day and night, allowing researchers to observe fluctuations in fasting blood sugar over morning periods.

What they found was significant variability both between individuals and within the same person on different days. On average, fasting blood sugar rose 0.2 mg/dL each year with age. But a person’s morning readings could vary by as much as 7.5 mg/dL solely due to natural day-to-day changes. With the threshold between normal and diabetic ranges a mere 26 mg/dL, this degree of fluctuation calls current diagnostic criteria into question.

The researchers then modeled how this variability could impact diabetes classification. Of the over 5,000 participants who appeared normal based on their first morning reading, 40% would have been reclassified as prediabetic and 3% as diabetic had their full glucose profiles been considered. Even accounting for two initial readings, about a third might have been misdiagnosed.

“Our findings challenge the traditional approach of relying on one or two isolated fasting blood glucose measurements,” says senior author Eran Segal. “Substantial natural variability exists, and given its impact on diagnoses, more comprehensive evaluations may be needed.”

While gold standard options like oral glucose tolerance tests are time-consuming and expensive, glucose monitors provide an accessible middle ground. Worn for just one week, they can yield valuable insights into an individual’s typical highs and lows. The researchers stress careful interpretation is key – a single abnormal reading may not definitively signal disease if sandwiched between many normal ones.

The study also helped uncover various ties between average fasting glucose and cardiometabolic health. Higher levels associated with increased weight, blood pressure, liver enzymes and smaller retinal vessels as well as disturbed sleep – reinforcing how glucose impairments may exert effects along a physiological continuum, not just between categorical labels.

Looking ahead, continuous monitoring data could inform personalized diagnostic criteria accounting for a person’s natural variation. It may also help streamline screening by identifying those requiring follow-up sooner. As technology improves to make these tools ever more convenient and affordable, Segal believes their insight into glycemic patterns will grow increasingly valuable for optimizing prevention and treatment strategies across the spectrum of blood sugar regulation.

Though more research is still needed, this work underscores the complexity glucose control and challenges a one-size-fits-all approach. By granting a high-resolution view of metabolic fluctuations, continuous tracking may hold the key to crafting more precise definitions of health and disease – and ultimately better care for all.


  1. Shilo, S., Keshet, A., Rossman, H. et al. Author Correction: Continuous glucose monitoring and intrapersonal variability in fasting glucose. Nat Med (2024).

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About the Author

  • Dilruwan Herath

    Dilruwan Herath is a British infectious disease physician and pharmaceutical medical executive with over 25 years of experience. As a doctor, he specialized in infectious diseases and immunology, developing a resolute focus on public health impact. Throughout his career, Dr. Herath has held several senior medical leadership roles in large global pharmaceutical companies, leading transformative clinical changes and ensuring access to innovative medicines. Currently, he serves as an expert member for the Faculty of Pharmaceutical Medicine on it Infectious Disease Commitee and continues advising life sciences companies. When not practicing medicine, Dr. Herath enjoys painting landscapes, motorsports, computer programming, and spending time with his young family. He maintains an avid interest in science and technology. He is a founder of DarkDrug

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